(a) Field of the Invention
This invention relates to a voice coil used with the speaker of acoustic apparatus such as a stereophonic device, and more particularly to a voice coil whose heat-resistant region has excellent thermal resistivity corresponding to the so-called H type.
(b) Description of the Prior Art
The voice coil is fabricated by applying a solvent to a self-fusible electrically insulated wire which is coated with baked adhesive paint with an insulating membrane interposed therebetween and regularly winding said wire around a paper tube (or a bobbin) covered with an adhesive layer, and laminating a plurality of said coil plies on each other.
As compared with other electric signal-voice conversion devices, the conventional voice coil has a far lower capacity to convert an input electric signal into a voice (realized by the mechanical vibrations of conical paper). The loss of an electric signal leads to the generation of heat, and consequently the temperature rise of the voice coil. Therefore, the voice coil tends to indicate a higher temperature as it is designed to be more reduced in size and produce a higher output. It is not rare that the voice coil reaches as high a temperature as 450.degree. to 500.degree. C., though for a short interval. The mechanical and thermal destruction of the voice coil first arises between the paper tube and surrounding wire coils, and then most prominently in the adhered interfaces between the wire coil plies as well as between the adhered turns of the respective coils. The main reason for this undesirable event is that (1) strains arise between the paper tube and surrounding wire coils because they have different thermal expansion coefficients; and (2) displacement occurs between the paper tube and surrounding wire coils due to the vibration of the voice coil assembly and the heat release of the wire coils. In the worst case, the wire coils are taken off the paper tube, thus resulting in the loss of the function of a voice coil.
With the conventional voice coil, however, a low boiling alcoholic solvent was applied for the dissolution of the adhesive layer coated on the self-fusible electrically insulated wires. Therefore, the main component of adhesive paint was necessarily limited to a synthetic resin having an aliphatic main chain, for example, a resin of the polyvinyl butyral series or alcohol-soluble polyamide series. Hitherto, therefore, there has been obtained only A or E class in heat resistance criterion which has a low heat resistance.
Recently a high output and high performance are demanded of acoustic apparatuses such as a stereoacoustic device. Consequently, the speaker used with said device undergoes a more rigid load, and a voice coil applied to the speaker is inevitably required to have prominent resistance to vibrations and heat. To meet the above-mentioned requirements, an adhesive paint prepared by blending various curing agents with an alcohol-soluble polyamide resin (the main component of the thermoplastic adhesive) has been applied to the self-fusible electrically insulated wire and paper tube. To-date, however, no voice coil has been proposed which fully satisfies the thermal properties.
With the conventional voice coils, the adhesive applied to the self-fusible electrically insulating wire and paper tube is mainly prepared, as mentioned above, from a synthetic resin containing a main chain of aliphatic series. The voice coil has been fabricated by solving and swelling the adhesive layer by means of a solvent of alcoholic series. Consequently it has been impossible to fabricate a superbly heat resistant voice coil whose heat-resistant is represented by the H class regarded as indispensable to a compact high output speaker.